I had problem with Cellular network showing for few minutes and then disappears. I flashed with flasher 3.5 even with new version P.R 1.3.1 n still it does not get solution. please help

Quick message...
Do you have 2g-3g Cellular network switcher? You could try to test whether this problem depends on the mode (2G, 3G, or dual).
Hoping somebody can help...
Best wishes.

It could be another case of faulty SIM contacts or cold joints under the GSM chips. There was a thread about that refreshed just recently, like a day or two ago.

The OP does not mention which phone this is but references to flasher 3.5 and PR 1.3.1 suggest N900.

Yeah, this is often a symptom of the beginning of the end of the telephony hardware working correctly.

Like pichlo said, there are two spots where it seems to be caused - the motherboard-to-telephony-chips connections, and the SIM card slot connections, becoming loose. The solution in both cases is either hot-air reflow or similar means to getting the connections heated to the point of melting so that they can re-solidify snugly again, or the hackish way of opening up the N900 and jamming springs between the telephony chips and the EM shield that covers them, and paper or something by the SIM card slot; that in turn applies enough pressure to the problematic spots to keep the normal jittering/movement of the phone in day-to-day use from separating the various contacts, which is what causes the loss of network in the first place.

So yeah. My main N900 currently employs both of these methods - many springs jammed in between telephony chips and their EM shield to keep the chips pressed against the motherboard, and several layers of paper jammed into the sim card holder (on top of sim card) and above the sim card holder (between it and the battery). First I did just the former, which held me over for a few weeks, then the problem started to surface again so I did the latter as well. Connection's been perfect again for the week since (with one or two exceptions, just a few seconds long).