Volkswagen transporter 1.9 diesel van r reg starts no problem but has no acceleration. When out of gear and press accelerater no response for about omne min then very slowly builds up revs can drive but need to go into higher gear than usual to get any speed if select lower gear slows down and need to change back up to slowly build up speed again. After couple hours clears and will run ok for days until not use it for few days and problem reappears have changed air and fuel filters. Recently changed glow plugs / starter hence good start could this be part of cause?